1. Field of the Invention
The present invention is directed generally to sensors and more particularly to sensors used to detect vibrations in rotary machines.
2. Description of the Prior Art
Turbine blades, because of their complex design, can suffer from vibrations at frequencies which correspond to natural frequencies of the blades called modes. Each mode is associated with a different type of vibration such as along the rotational axis of the turbine, perpendicular to the rotational axis of the turbine, etc. In order to prevent excessive vibration of the blade about its normal position, normal design practice dictates that the blades be constructed such that these modes are located between harmonics of the operating frequency of the steam turbine. However, manufacturing tolerances, changes in blade attachment to the rotor, changes in blade geometry due to erosion and changes in the operating frequency of the turbine, among other factors, cause mode frequencies to approach harmonics of the operating frequency.
The approach of the modes to the harmonics of the operating frequency may result in physical damage to the steam turbine. When the amplitude of the vibration exceeds a certain level, objectionable stresses are set up in the blade. If the condition is not detected and remedied, the blade may eventually fracture resulting in an extremely costly forced outage of the machinery. Thus, a method for detecting this vibration is necessary in order to prevent such damage.
Historically, the vibrational modes of steam turbine blades have been measured by placing strain gages on the rotating blades and telemetering the information to a stationary receiver. This method suffers from three significant drawbacks. First, the strain gage has a very short life due to erosion caused by steam passing through the turbine blades. Second, each blade requires a strain gage if all blades in a row are to be monitored. Third, the complexity of continuously and reliably supplying power to the strain gage and transmitting the signal reliably from the rotating rotor disk to a stationary receiver creates severe difficulties. For these reasons, other types of sensors have been investigated.
The present application is related to co-pending U.S. application Ser. No. 205,770 entitled APPARATUS FOR PRECISE DETECTION OF BLADE PASSING TIMES filed June 13, 1988, now U.S. Pat. No. 4,922,757 and assigned to the same assignee as the present invention. U.S. application Ser. No. 205,770 is directed to an apparatus for detecting the passing of the blades of a rotating machine past a stationary sensor. The sensor produces an output signal each time a blade passes the sensor. A zero crossing detector produces an output signal each time the input signal crosses a reference axis. A phase shifter shifts the phase of the input signal to produce a gating signal coinciding with the expected arrival time of the blade at the sensor. A gating device is responsive to the gating signal for conducting output signals which occur during the expected arrival time of the blade at the sensor.
The invention disclosed in U.S. application Ser. No. 205,770 requires that the circuitry be calibrated to operate in conjunction with any particular sensor that is used with the apparatus. This calibration is necessary both because of the different characteristic signal amplitudes associated with any particular sensor and because the signal amplitudes also depend upon the distance between the turbine blade tip and the surface of the sensor. In addition, this apparatus is sensitive to the frequency of the input signal and thus must be designed for signals of any one particular frequency range.
Thus, there is a need for a turbine blade arrival time processor which can compensate automatically for differences in input signal amplitude and is not sensitive to input signal frequency.